This invention relates to means for effecting connections between surfaces, and more particularly to means for establishing a fire resistant connection and seal between surfaces of the type that are known to exist in wellhead and valve equipment.
The fact that extreme service conditions are encountered in wellhead applications has long been recognized. Moreover, it has long been known that the nature of such extreme service conditions encompasses, by way of example and not limitation, conditions such as the presence of high and low temperature, sour gas, high fluid velocity, pressure cycling, thermal shock, and/or the existence of forces of vibration, bending, compression, tension or any combination of these forces. In an effort to provide equipment that would be suitable for employment in such wellhead applications, i.e., that would successfully withstand being subjected to extreme service conditions of this type, metal-to-metal seals have heretofore been employed for purposes of effectuating connections and seals in equipment designed to be used in wellhead applications of the aforedescribed type. This selection of metal-to-metal seals for use in this manner has been influenced to some extent by environmental and economic considerations. Moreover, the metal-to-metal seals that have actually been selected for use for this purpose have been of various designs. By way of illustration, reference may be had, among others, to U.S. Pat. No. 2,766,999, which issued on Oct. 16, 1956 to J. D. Watts et al., and/or to U.S. Pat. No. 4,214,763, which issued on July 29, 1980 to R. E. Latham for a showing of a metal-to-metal seal that is disclosed to be suitable for use in equipment, which is designed for employment in wellhead applications.
Although these earlier types of metal-to-metal seals when employed in equipment designed for use in wellhead applications have proven generally to be capable of withstanding the extreme service conditions associated with such applications, i.e., conditions of the sort that have been enumerated hereinbefore, these metal-to-metal seals were never intended to be fire resistant. That is, no requirement existed insofar as the design of these metal-to-metal seals was concerned that they embody the capability of maintaining sealability during periods of thermal expansion and contraction occasioned by the occurrence of wellhead fires. It is only more recently that the matter of fire resistance has come to be viewed as a consideration in the design of connections and seals of the type found in equipment that is intended for use in wellhead applications. Moreover, to some in the industry this matter of fire resistance has gone beyond the state of being simply a consideration, but rather has now risen to the level of being a requirement that future designs of metal-to-metal seals must satisfy.
For purposes of exemplifying what constitutes fire resistant wellhead equipment as this term is being employed herein, reference can be had at least insofar as 5,000 psig and 10,000 psig working pressure equipment is concerned to the statement of requirements that is embodied in American Petroleum Institutes's RP6F "Modified". As set forth therein, 5,000 psig working pressure equipment must satisfy the following test criteria: flame temperature one inch from the wall--1100.degree. C. (2000.degree. F.); stabilization temperature within 31/2 hours--650.degree. C. (1200.degree. F.); high test pressure throughout the test--3750 psi; low test pressure throughout the charging test--500 psi; test media-water; hold period at stabilization temperature--one hour; valve backseat test pressure for oil service--100 psi; valve backseat test pressure for gas service--500 psi; allowable leakage-zero external leakage; and functional valve test after burn-replace stem assembly, open one time, zero external leakage allowed. With respect to 10,000 psig working pressure equipment, the test criteria that must be satisfied are as follows: flame temperature one inch from the wall--1100.degree. C. (2000.degree. F.); stabilization temperature within 31/2 hours--650.degree. C. (1200.degree. F.); high test pressure throughout the charging test--7500 psi; low test pressure throughout the test--500 psi; test media--water; hold period at stabilization temperature--one hour; valve backseat test pressure for oil service--100 psi; valve backseat test pressure for gas service--500 psi; allowable leakage--zero external leakage; and functional valve test after burn--replace stem assembly; open one time, zero external leakage allowed.
The high temperatures which are encountered during wellhead fires give rise to a variety of problems. Included among these are problems that can be linked to the rapid thermal heatup and cooldown of the material which is exposed to the wellhead fire, the expansion and/or contraction of the exposed material, and/or a loss in the properties which the exposed material exhibits. For ease of classification, however, the aforereferenced problems fall basically into two categories. Namely, there are those problems which relate to the structural characteristics exhibited by the wellhead equipment material upon being exposed to a wellhead fire, and there are those problems that relate to the capability of connections and seals in wellhead equipment to maintain their sealability when the wellhead equipment is subjected to a wellhead fire.
Addressing first the matter of the structural characteristics of wellhead equipment material, for purposes of rendering such material fire resistant, i.e., capable of satisfying the test criteria enumerated above for 5000 psig and 10,000 psig working pressure equipment, the loss of tensile strength exhibited thereby when exposed to a wellhead fire can be compensated for in several ways. First, advantage can be taken of the fact that API's RP6F "Modified" permits a twenty-five percent downrating to be had in the pressure limits which 5000 psig working pressure equipment must be capable of withstanding. Secondly, the pressure vessel walls of the equipment in question can be oversized. Accordingly, it has been found that this twenty-five percent downrating permitted by API's RP6F "Modified" coupled with the oversizing of the pressure vessel walls of the wellhead equipment is sufficient to compensate for the loss of the tensile strength that occurs when the wellhead equipment is exposed to elevated temperatures.
Although wellhead housings and valve housings become large when the walls thereof are oversized, i.e., when API type materials are employed therefor, such housings nevertheless remain within practical limits. Therefore, there is no necessity to make use of exotic steels, etc. for this type of equipment. This is not to say, though, that future developments in the area of materials research may not produce new cost effective, high strength alloys, which will enable a reduction to be had in the sizing of wellheads and valves of the type that fall within the category of 5000 psig working pressure equipment.
Turning now to the matter of the sealability of the connections and seals that are embodied in wellhead equipment, it is essential for the reasons that have been discussed previously herein that such connections and seals be effectuated through the use of metal-to-metal seals. On the other hand, however, if such metal-to-metal seals are to be capable of exhibiting adequate tensile strength at elevated temperatures the view has been taken that there must be utilized therein high strength materials as overlays or seal ring materials. Elastomers, as they are known today, are known to perform unsatisfactorily when employed under the sort of conditions to which wellhead equipment is subjected when a wellhead fire occurs. The one nonmetallic material which may have some merit for use in such applications is that which is referred to by those in this industry as "graphoil".
By and large, therefore, it can thus be seen that in order to develop wellhead and valve equipment that is fire resistant, i.e., satisfies insofar as the principal requirements for fire resistance are concerned the statement of requirements that is embodied in API's RP6F "Modified", a need has existed to develop improved sealing techniques that would be suitable for use to effect seals that would maintain their sealability at elevated temperatures. More specifically, there has existed a need to develop improved high temperature sealing techniques that would be applicable for use in connection with both the tubular and annular seals that are to be found in wellhead equipment, and which would enable the latter equipment to withstand in terms of sealability the range of temperatures to which such equipment would commonly be exposed in the course of a wellhead fire. In this context, in order to develop such an improved high temperature sealing technique there would exist a need to address the following areas: the thermal and metallurgical characteristics of the materials involved, the relative movement that occurs between the mating parts, and the sliding action that the seal must endure.
Attempts at the development of such high temperature sealing techniques have been undertaken. Further, the focus thereof at the outset was predicated on the following notions. Foremost was the notion that a clamp connection because of its mass would prove to be beneficial insofar as a fire resistant connection is concerned. Moreover, it was felt that such a clamp connection would probably prove to be the only suitable connection for use in wellhead and valve equipment that was designed to be fire resistant. Accordingly, considerable time and effort was devoted to the development of a suitable clamp connection that would maintain its sealability at elevated temperatures. However, not only did the mass of a large enhanced clamp prove to be detrimental to heat exchange properties of the wellhead and valve equipment, per se, but indeed proved to be uncontrollable in terms of torsional deflection and permanent set. In turn, the latter prevented retention of any seal that was dependent upon the clamp connection as a holding device.
As a result of the realization of the above, the development of a studded clamp connection was undertaken. However, the unfavorable heat transfer properties of the added mass of the large clamp soon led to the abandonment of the clamp itself. This was done principally so that a more favorable heat transfer could be realized in a less irregular surface surrounding the wellhead housing. It was then concluded that in the context of attempting to render wellhead and valve equipment fire resistant large clamp connections should not be utilized.
Enhanced flange connections have since been developed which are capable of maintaining the required seal contact force and connection stability. Furthermore, it is practical to prepare such an enhanced flange connection with a hub profile that may be utilized during the drilling operation. Notwithstanding this though it is still strongly recommended that large clamps not be utilized in wellhead and valve equipment that is intended to be designated as being fire resistant.
Thus, to summarize, it has been concluded from analytical and test results that API type materials are suitable for use in forming pressure containing members of wellhead housings, valve bodies, and bonnets. Further, it is viewed as being practical to construct valve bodies and wellhead housings of such materials. That is, the use of such materials for this purpose does not lead to enormous enlargement of the equipment to the point of being impractical. On the other hand, however, it must be recognized that API type materials are not suitable for use in performing a sealing function. Accordingly, it is essential that within any wellhead and valve equipment housings that overlays and seal rings of high strength materials be inserted. Furthermore, these overlays and seal rings of high strength material must be of sufficient size and integrity to withstand the loading forces necessary to effect the sealing function. In addition, the materials utilized in this connection in the overlays and in the seal rings must of necessity be selected for compatibility, for their elevated temperature strength, and of great importance, their thermal conductivity. Namely, it is very important that the material selected for use in these sealing areas be compatible from the standpoint of thermal expansion and contraction, corrosivity, weldability and gall resistance. However, even when the above criteria have been satisfied, there still remains a need to provide a high temperature seal, which in terms of its design as contrasted to the matter of the materials from which it is formed, is suitable for use in wellhead and valve equipment that may be subjected to elevated temperatures of the type that are experienced during the course of a wellhead fire. That is, a need has been evidenced for a seal design wherein a seal constructed in accordance therewith would when employed in wellhead and valve equipment be characterized by the fact that it possessed the capability of maintaining its sealability even when the wellhead and valve equipment in which it was embodied was involved in a wellhead fire.
In particular, a need has been evidenced for a seal embodying a design wherein a characteristic thereof is the fact that a positive locking action exists between the seal ring of the seal and the mating sealing surfaces that engage the seal ring. More specifically, it is desired to provide such a seal wherein by virtue of the fact that the seal ring of the seal is locked in place relative to the mating sealing surfaces, the seal is capable of responding to thermal movement such that both the seal ring and the mating sealing surfaces which are in engagement therewith remain in contact even though thermal distortion may occur during thermal cycles, and in particular thermal cycles of the type that are encountered when a wellhead fire occurs.
It is, therefore, an object of the present invention to provide a new and improved connection, i.e., seal, suitable for employment in wellhead and valve equipment.
It is another object of the present invention to provide such a connection, i.e., a seal, which when employed in wellhead and valve equipment is capable of withstanding the conditions imposed thereupon during the occurrence of a wellhead fire.
It is still another object of the present invention to provide such a connection, i.e., a seal, which is characterized in that it exhibits adequate tensile strength even at the elevated temperatures that exist when a wellhead fire occurs.
A further object of the present invention is to provide such a connection, i.e., a seal, which is characterized in that it exhibits the capability of being able to maintain its sealability even at the elevated temperatures that exist when a wellhead fire occurs.
A still further object of the present invention is to provide sealing means of unique design which is particularly suited for use in a connection, i.e., a seal, of the sort that is intended for employment in wellhead and valve equipment of the type that is designed to be denoted as being fire resistant.
Yet another object of the present invention is to provide sealing means in the form of a seal ring having a dovetail-shaped portion which when employed as a component of a connection, i.e., a seal, in wellhead and valve equipment is operative to enable the connection, i.e., the seal, to maintain its sealability when the equipment in which the connection, i.e., the seal, is embodied is subjected to the conditions that are associated with the occurrence of a wellhead fire.
Yet still another object of the present invention is to provide such a connection, i.e., a seal, embodying such a seal ring having a dovetail-shaped portion which is relatively inexpensive to provide and easy to employ, while yet being capable of providing reliable and effective service even when exposed to the conditions that exist when a wellhead fire occurs.